Campbell was born Beatrice Stella Tanner in Kensington, London, to John Tanner and Maria Luigia Giovanna, daughter of Count Angelo Romanini. She studied for a short time at the Guildhall School of Music.
Her first marriage, from which she took the name by which she is generally known, produced two children, Beo and Stella, and ended with the death of her first husband in the Boer War in 1900.
She was well-known as an amateur before she made her stage debut in 1888 at the Alexandra Theatre, Liverpool, four years after her marriage to Patrick Campbell. In March, 1890, she appeared in London at the Adelphi, where she afterward played again in 1891–93. She became successful as a result of starring in Sir Arthur Wing Pinero's play, The Second Mrs Tanqueray, in 1893, at St. James's Theatre where she also appeared in 1894 in The Masqueraders. As Kate Cloud in John-a-Dreams, produced by Beerbohm Tree at the Haymarket in 1894, she made another success, and again as Agnes in The Notorious Mrs. Ebbsmith at the Garrick (1895). Among her other performances were those in Fédora (1895), Little Eyolf (1896), and her notable performances with Forbes-Robertson at the Lyceum in the rôles of Juliet in Romeo and Juliet, Ophelia in Hamlet, and Lady Macbeth (1895–98) in the Scottish play. Despite her marriage, she continued to use "Mrs Patrick Campbell" as her stage name.
In 1900, Campbell made her debut performance on Broadway in New York City in Magda, a marked success. Subsequent Broadway roles included The Joy of Living (1902), as Melisande to the Pelleas of Sarah Bernhardt in Pelléas et Mélisande (1904), The Whirlwind and The Bondman (1906), Hedda Gabler (1907), The Thunderbolt (1908), Lady Patricia (1911), Bella Donna (1911), and Shaw's Pygmalion (1914). She would return to perform there on a number of occasions until 1930.
In 1914, she played Eliza Doolittle in the original production of Shaw's Pygmalion; though much too old for the part at 49, she was the obvious choice, being by far the biggest name on the London stage, and Shaw would have seen it no other way since he wrote the play for her in particular.